Pulse converter for exhaust system

ABSTRACT

A silencing system for the exhaust gas of internal combustion engines or the like has means to convert high energy exhaust pulses into more or less standard pulses which comprise compliance devices located at points of minimum compliance and inertance devices at points of maximum compliance.

United States Patent [191 Bergson et a1;

[ PULSE CONVERTER FOR EXHAUST SYSTEM [75] Inventors: Arnold A. Bergson, Glendale, Wis;

Paul A. Johns, Grass Lake, Mich.

[731 Assignee: Tenneco Inc., Racine, Wis.

[22] Filed: Mar. 14, 1973 [21] Appl. No.: 340,966

[52] US. Cl. 181/35 B, 181/33 D, 181/36 D, 181/53 [51] Int. Cl. F0ln 1/00 [58] Field of Search 181/33 D, 33 F, 35 B, 36 D, 181/47 R, 41, 53; 138/26; 417/540; 60/312 [56] References Cited UNITED STATES PATENTS 1,910,672 5/1933 Bourne '181/35 B UX [451 Apr. 30, 1974 2,765,044 10/1956 Hatte 181/41 X 2,936,041 5/1960 Sharp et a1. 181/33 D UX 2,943,641 7/1960 Arnold 181/41 UX 3,338,331 8/1967 Jettingoff 181/35R Primary Examiner-Richard B. Wilkinson Assistant Examiner-John F. Gonzales Attorney, Agent, or Firm-Harness, Dickey & Pierce ABSTRACT I A silencing system for the exhaust gas of internal combustion engines or the'like has means to convert high energy exhaust pulses into more or less standard pulses which comprise compliancedevices located at points of minimum compliance and inertance devices at points of maximum compliance.

46 Claims, 12 Drawing Figures BACKGROUND OF THE INVENTION All internal combustion engines generate a series of discrete exhaust pressure pulses whose basic characteristics of peak amplitude, waveshape, and repetition rate depend on thetype of engine, the speed and the load. Conventional 4 cycle reciprocating piston engines of the type ordinarily used in automobiles, trucks, etc. exhibit a field of pulses having,- from the standpoint of sound attenuation, relatively desirable characteristics in that the waveshapes are relatively symmetrical or sinusoidal, the peak to average pressure ratios are moderate, and the higher harmonic amplitudes decrease rapidly with increases in frequency. In general, the exhaust gas discharged by these piston engines has been satisfactorily silenced by apparatus derived from classical methods of analysis assuming a linear sound field.

Other engines, however, such as the Wankel engine, and probably rotary piston engines generally and twocycle reciprocating piston engines, generate high energy pulses which appear to be N-shaped, with extremely high peak and peak to average pressures, with very unfavorable dissymmetries causing a flat frequency-pressure response, and an unusually large number of acoustically significant notes as much as one hundred or more in one pulse. In the case of the Wankel engine, the overall noise level is considerably higher than for a comparable piston engine and the nodal/antinodal characteristics of the exhaust system conduits virtually disappear. Thus, the usual standing wave theory of exhaust silencing is not satisfactory and the Wankel engine has presented a significantly new exhaust gas silencing problem requiring a new approach for solunon.

Additionally, some 4 cycle reciprocating piston engines, particularly large displacement engines having large exhaust ports which open fully at a rapid rate,

present a similar though less severe problem in sound attenuation which is not practically solved by conventional silencing means.

BRIEF SUMMARY OF THE INVENTION It is. the purpose of the invention to provide an exhaust gas silencing system for Wankeltype rotary piston engines and for other internal combustion engines or fluid propelling units that dischargehigh energy or shock-like gas pulses which are very difficult to attenuate by means of normalmuffling devices.

The invention is based on a recognition of the different nature of the Wankel-type exhaust and provides a means to convert that pulse into one that is similar to the pulse discharged by a typical four-cycle reciprocating piston engine. In doing this, points of maximum and minimum relative compliance forfthe exhaust system are determined and, commencing at the upstream portion, pulse conversion is obtained by placing acoustical inertance means at the points of maximum compliance and acoustical compliance means at the points of minimum compliance. Thus, generally, the invention provides a pulse converter in the form of a relatively small and simple acoustic device for insertion in the upstream portion of the exhaust system to convert unfavorable engine exhaust pressure pulses into more favorable pulses downstream of the pulse converter. This 2 permits the remaining downstream acoustic devices, if required, to be of a relatively conventional type.

DESCRIPTION OF THE DRAWINGS FIG. I is a partly schematic, partly sectional view of a Wankel engine having an elementary exhaust system utilizing general principles of this invention;

FIG. 2 is a graph or map of the pulse pressure along the length of a Wankel engine exhaust conduit;

FIG. 3 is a graph partly broken away showing the relative acoustic compliance at different points along the length of a Wankel engine exhaust conduit, the conduit length being 150 inches;

FIG. 4 is an improved form of pulse converter as compared with the device of FIG. 1;

FIG. 5 is a side elevation, partly broken away, of an exhaust system for a Wankel engine that contains a modified form of pulse converter;

FIG. 6 is an enlarged longitudinal cross section of the pulse converter shown in FIG. 5;

FIG. 7 is a schematic cross section through a typical silencer that may be used in the system of FIG. 5 to attenuate the converted pulse;

FIG. 8 is a longitudinal cross section through another modified form of pulse converter;

FIG. 9 is a longitudinal cross section through a still further modified form of pulse converter;

FIG. 10 is a longitudinal cross section through another modified form of pulse converter;

FIG. 11 is a longitudinal cross section through the pulse converter of FIG.. 10 taken at right angles to the section of FIG. 10; and

FIG. 12 is a cross section along the line l2-l2 of FIG. 11.

In the drawings, X s indicate spotwelds or the equivalent.

DESCRIPTION OF THE INVENTION FIG. 1 shows a Wankel engine 1 that has an exhaust port 3 which discharges gas into an exhaust conduit 5 that is a part of an exhaust silencing system 7. The conduit 5 conducts the gas to a pulse converter 9 in the system 7 which is in the form of a pair of shells or housings l1 and 13 which provide two empty chambers or volumes l5 and 17 that form acoustic compliances l9 and 21, respectively. The volumes l5 and 17 are connected by a conduit or pipe 23 which is preferably smaller in diameter than the pipe 5 and forms an acoustic inertance interconnecting the compliances 19 and 21. The combination of compliances l9 and 21 with the inert ance 23 form the pulse converter 9. The outlet pipe 25 fordevice 21 may, if required, conduct the gases to conventional acoustic controls 27 and thereafter discharge the gas to atmosphere through the tailpipe 29. If properly located in the system 7 in accordance with the invention, the pulse converter 9 changes the characteristics of the exhaust pulses received from the engine l in such a way that the pulses discharged into pipe 25, if not completely attenuated, are of a shape and amplitude similar to that usually obtained from four-cycle, six or eight cylinder piston engines. These pulses can be attenuated to a desired low noise level by a muffling device 27 that is similar to those already known in the field.

The pulse emitted by the rotary (Wankel) engine 1 is extremely difficult to handle from the standpoint of sound attenuation. Conventional automotive exhaust sound engineering methods based on low pressure, linear sound field acoustics, standing wave or nodal sound frequency characteristics, and sinusoidal wave patterns are not sufficient to produce practical sound attenuati ing devices. The Wankel pulse contains in the order of l l-l20 individual high pressure, noise producing frequencies whereas the average four cycle, six or eight cylinder engine damps out many frequencies and its exhaust noise spectrum contains only three or four (or less) high energy frequencies (usually low) along with several low pressure higher frequencies. The Wankel pulse is many times more powerful, being about 200 decibels (over l0 psi) peak pressure. At some engine conditions it may fill the entire length of the exhaust system. A rough comparison is that the Wankel pulse corresponds to hanging down simultaneously all keys of a piano while the typical four cycle six or eight cylinder piston engine pulse corresponds to striking four or five keys at the lower end of the keyboard.

FIG. 2 illustrates a pressure map taken at various points along the length of a 2 inch O.D. straight pipe. 150 inches long, attached to the exhaust manifold of a two rotor, 60 OLD, 100 HP. Wankel engine at 2,500 rpm, inches Hg. The line 31, representing the pressure readings along the length of the pipe, does not display the normal nodal structure or shape obtained with conventional four cycle piston engines. The pulse has maximum pressures much higher than conventional engines and it substantially fills the pipe. It is more or less N-shaped and has a sharp high frequency leading edge that seems to act almost like a drill as the high pressure wave travels down the pipe rotating like a projectile. It is believed that similar pulse characteristics will be obtained with other rotary engines and with two cycle piston engines.

In normal engineering of an exhaust system to achieve silencing, the nodal characteristics of the pipe are determined and silencing devices are arranged in accordance with this information to attenuate the few troublesome notes as per the standing wave theory. The rotary engine pulse characteristics preclude this approach and require another. In accordance with the present invention, openings are formed in a standard diameter exhaust system conduit at numerous regular intervals along its length and probes are mounted on the pipe at the openings. The pipe is attached to exhaust manifold and the engine operated, preferably, under the worst silencing conditions that are encountered in normal operation of the engine (e.g., when the pulse fills the entire pipe). A probe microphone is attached in turn to each of the spaced openings or stations to obtain pressure pulse oscilloscope photos. Each of these gives a peak pressure reading in terms of microphone voltage and a pulse width in terms of milliseconds of time. By comparing the readings at successive stations, pulse peak voltage (or pressure) and pulse width for each station is obtained. The change in pressure between two adjacent stations and the change in pulse width between two adjacent stations is useful data, characteristic of the engine being studied. The ratio of pipe compliance to pulse flow between adjacent stations is proportional to the ratio of the change in pulse width to the change in pulse pressure between these stations. Therefore, the readings taken reveal the relative compliance at the various stations along the length of the exhaust system.

For the engine mentioned, FIG. 3 shows a schematic plot or graph made of relative compliance determined in this way at various stations along a 150 inches length of a uniform 2 inch diameter exhaust conduit and, therefore, indicates the variation of compliance as a function of pipe length in terms of distance from the open or outlet end of the system. Compliance had major high peaks at about and 120 inches and major low peaks at about 60 inches, inches, and 145 inches and therefore showed fairly regular spacing of about 40 inches between adjacent peaks. At the low peaks where compliance is a minimum, the pipe appears to the pulse as being necked down or reduced in diameter. At the high peaks where the compliance is a maximum, the pipe appears to be enlarged. In accordance with the invention, volume sections or compliances, such as elements 19 and 21, in FIG. 1, are in: serted in the exhaust system at points of minimum compliance and an inertance is inserted in the system at one or more points of maximum compliance. For example, the exhaust system 7 of FIG. 1 may be used with an engine having the characteristic of FIG. 3 by locating the volume or compliance 19 so that its longitudinal midpoint is approximately at the 60 inches mark of FIG. 3, and the compliance 21 so that its midpoint is approximately at the next point of minimum compliance, i.e., I00 inches. The connecting pipe 23 between the elements l9 and 21 is necked down or reduced in diameter from the section 5 and, therefore, serves as an inertance and extends across the peak compliance point 33 located between the 60 and 100 inches minimum compliance points and its midpoint is approximately at the 80 inches high peak.

In an actual test with a two rotor, 60 C.I.D. 100 hp Wankel engine having a 2 inch pipe 5, the elements 19 and 21 were located, as just mentioned, and increasing attenuation was obtained as the uniform diameter of pipe 23 was decreased to l /2 inches, l A inches, and 1 inch, the elements 19 and 21 being 3 /2 X 6 /a uniform oval shells of about l0 inches length, the smaller diameter increasing the inertance. Equally good attenuation with a l /2 inch pipe 23 between volumes and less backpressure was obtained by using three volumes (at about 60 inches, 100 inches, and 145 inches) and two connecting pipes.

FIG. 4 illustrates an improved form of pulse con-. verter 51 that was satisfactorily used to silence an engine 1 having pulse characteristics substantially as shown in FIG. 3. In this arrangement, two open equal volume compliances 53 and 55 are connected, respectively, by conduits or minor inertances 57 and 59 to double volume major inertance device 61. Compliances 53 and 55 had their midpoints located at about 100 and 145 inches from the open end of the system and inertance 61 had its midpoint located at about inches. Each of the elements 53, 61, and 55 comprises an outer shell 63 and end headers 65 and 67 interlocked, with the ends of the shell 63 to form the internal volume, the headers having suitable necks 69 for attachment to the various conduits. Additionally, the device 61 has an internal partition 71 that subdivides it into equal volumes 73 and 75. The partition has a neck 77 in which is, at its midpoint, mounted a conduit 79 that is preferably the same length as a chamber 73 or 75 and aligned with conduits 57 and 59 and preferably of the same diameter. Satisfactory attenuation was obtained when the diameter of these pipe sections (57,

59, and 79) was 1 /2 inches as compared with 2 inches for the inlet and outlet pipes 81 and 83 and the cans 53, 55, and 61 were all inch long ovals of3 /2 inches X 6 /2 inches.

FIGS. 5, 6, and 7 show a complete rotary engine exhaust system that embodies principles of the invention. In FIG. 5, the Wankel (or rotary) engine 101 has a manifold 103 conducting exhaust gases to a single outlet 105 to which is attached the inlet end of an exhaust pipe 107 for the exhaust system 109. The pipe 107 is clamped to the inlet bushing of a substantially conventional silencing device 111 which comprises an outer shell 1 13 and a perforate straight through gas flow tube the inlet bushing 121 of a pulse converter 123 embodying the present invention. The converter 123 has an outlet opening 125 at its downstream end in a sidewall; and suitably attached to the converter to receive gas from the opening 125 is another connecting pipe 127 which has a kick-up or arch portion 129 to go over the rear axle of a vehicle and conduct gas to the inlet of a substantially conventional type sound attenuating muffler 131. The unit 131 that is illustrated is one designed for use in the exhaust system of a current American made small piston engine automobile. The muffler 131 discharges gas into a tailpipe 133 that has its outlet open to atmosphere to form the open end of the system 109.

The pulse converter 123, as seen best in FIG. 6, is similar to that of FIG. 4 but is all contained within a single outer housing. It comprises a tubular outer shell 135 which is preferably oval in cross section, for example, being a 3 inch X 4 inch oval with an overall length of about 45 inches. The opposite ends of the shell 135 are closed by inlet and outlet headers 137 and 139, respectively, which are interlocked with the ends of the shell in rolled joints 141. Additional reinforcement for ends of the shell to resist the effects of the extremely high pulse pressures is provided by transverse partitions 143 and 145 at the upstream and downstream ends that have annular flanges spotweldcd to the shell. The

' header and partition 137 and 143 have aligned annular necks in which the inlet bushing 121 is supported'and spotwelded. An angle shaped reinforcing ring 147 is welded at a midpoint of an upstream unsupported length of the shell. A series of longitudinally separated transverse partitions 149, 151, 153, 155, and 157 are disposed within the shell 135 and have annular flanges welded to the shell. The partitions subdivide the interior of the shell into chambers 16 1, 163, 165, 167, 171, and 173. The partitions 149, 151, 153, 155, and 157 have aligned annular necks 177 which serve to support the imperforate tubes 179, 181', and 183. These tubes and the inlet and outlet tubes 121 and 127 are preferably of the same internal diameter, a feature that minimizes backpressure. The tube 179 acts as a minor inertance connecting chambers 161 and 163; tube 181 acts with adjacent chambers 165 and 167 as a major inertance 184; and tube'l83 acts as a minor inertance connecting chambers 167 and 173. Chamber-s 161 and 173 are preferably the same volume and act as major compliances. Chambers 165 and 167 are preferably the same size and tube 181 is preferably the same length as a chamber 165 or 167 and mounted at its midpoint. Chambers 163 and 171 are preferably dead, i.e., have no connection with gas flowing through the unit. The unit 123 is sized and connected so that their midpoints are located at (or as close as possible to) low compliance peaks (e.g., 60 and 100 inches of FIG. 3) and the midpoint of inertance 184 at a high compliance peak (e.g., 80 inches in FIG. 3).

Gas leaves the pulse converter 123 through the connecting pipe 127 and, as a result of the action of the pulse converter, the pulse shape is changed from the shock wave N-shape as a result of attenuation of many high frequencies to a pulse shape that is substantially symmetrical and conventional and, therefore, capable of attenuation by a silencing device embodying structure of a type generally known in the art, such as the muffler 131 (from another automobile) shown in FIG. 7. The muffler 131 has an oval outer shell 191 which is closed at opposite ends by headers 193 and 195, the header 193 having a neck 197 to support an inlet bushing 199 and header 195 having a neck 201 to support an outlet bushing 203. The space inside of the shell 191 is subdivided into longitudinally separated chambers 205, 207, 209, and 211 by transverse partitions 213,

. 215, and 217. The partitions 213 and 215 have aligned necks 219 to receive the end of the inlet'bushing 199 and the end of a perforate inlet tube 221, one end of the inlet tube being supported in the bushing 199. The inlet tube 221 has a louver patch 223 surrounded by a shell 225 to form a spit chamber for roughness and high frequency sound. It also has a louver patch 227 opening into chamber 207 to attenuate medium and high frequencies. A return flow tube 229 connecting chambers 209 and 205 is perforated along its length as indicated by louvers 231 to open into chamber 207, the tube being supported in aligned necks 233- in the partitions 213 and 215. An outlet tube 235 has its inlet end supported in a neck 237 in the partition 213 and its outlet end supported inside of the outlet bushing 203. The outlet tube 235 has a louver patch 239 opening into the chamber 207 to attenuate medium and high frequencies and higher frequencies and roughness maybe attenuated by a spit chamber 241 provided by the shell 243 that extends around the tube 235 to surround louver patches 245, the shell extending through and being supported in the partitions 215 and 217. An imperforate tuning tube 247 connects chamber 205 with chamber 211, forming the only inlet and outlet to chamber 211, whereby itsarea and length may be coordinated with the volume of chamber 211 to attenuate a low frequency. Openings 249 in inlet bushing 199 connect gas with chamber 205 to provide some roughness and high frequency attenuation.

Gas entering the inlet bushing 199 of muffler 131 flows along the length of pipe 221 into crossover chamber 209 where there is a reversal of flow as the gas enters pipe 229 to flow back to chamber 205. In chamber 205 flow is again reversed and as leaves the muffler by way of the outlet tube 235. In addition to attenuation already mentioned, the chambers 205 and 209 act with the connecting pipes to attenuate low frequencies that may have passed through the converter 123.

Thus, in the system shown in FIGS. 5 7, pipe ring and the highest frequencies are removed by the device 111 and the high energy pulse or shock waves are converted by pulse converter 123 into relatively favorable wave forms and shapes so that final attenuation by conventional standing wave and classical acoustical theory may take place in a suitable muffler or acoustic control such as the tri-flow device 131. Actually, the medium and high frequency attenuation by converter 123 is quite complete and the primary function of device 131 is to attenuate low frequencies and help lower the overall noise level.

FIG. 8 shows a pulse converter 301 functionally the same as that of FIG. 6 but of a slightly modified physical construction. In device 301 the common housing is eliminated and an assembly of modules or components is utilized. Thus, there are three modules 303, 305, and 307 in the form of tubular outer shells 309 closed at each end by welded headers 311 and strengthened by welded end partitions 313. The modules 303 and 307 have internal transverse reinforcement rings 315 (like ring 147 of FIG. 6) and module 305 has a center partition 315'. The device 303 has necks 316 in the header 311 and partition 313 to support an inlet bushing 317 for attachment to a pipe such as 107 or 119. The device 307 has a side outlet bushing 319 for attachment to pipe 127 or the like. The modules 303 and 305 are connected together by pipe 321 which is supported in and secured to aligned necks 323 in the adjacent headers and partitions. The modules 305 and 307 are connected'together by a pipe 325 which is supported in and secured toaligned necks 327 in the adjacent headers and partitions. An imperforate inertance tube 329 of the same length as the chambers on either side of partition 315 is supported at its midpoint in a neck 331 in partition 315 at the midpoint of unit 305.

As in the case of pulse converter 301, the devices 303 and 307 serve as compliances and the device 305 as an inertance.

A compact pulse converter 401 is illustrated in FIG. 9. In this modification there is an oval outer shell 403 whose opposite ends are closed by inlet and outlet headers 405 and 407 and reinforced by transverse partitions 409 and 411. An inlet bushing 413 is supported in and secured to aligned necks 415 in the header 405 and partition 409 and an outlet bushing 416 is supported in and secured to aligned necks 417 in header 407 and partition 411. Transverse partitions 419 and 421 subdivide the interior of the shell 403 into longitudinally separated chambers 423, 425, and 427. Partitions 419 and 421 have three pairs of aligned necks 429, 431, and 433. Necks 429 receive and support an inlet tube 435; necks 431 receive and support an enlarged shell 437 closed at opposite ends by headers 439 to form a return flow and inertance device 441; and necks 433 receive and support an outlet tube 443. The inlet tube 435 has a downstream end 445 that is bent through a smooth. non-restrictive 90 angle so that the end of the tube may open into an end of the shell 437 as seen at 447, being suitably secured in a gas tight joint at that point. Similarly, the upstream end of the outlet tube 443 is bent through a smooth, non-restrictive 90 angle as seen at 449 to open into the other end of the shell 437 as seen at 451, also being suitably secured to the shell in a gas tight joint. The shell 437 has a transverse partition 453 at its midpoint and this has a neck 455 in which is mounted, at its midpoint, an imperforate tube 457 of the same length as one of the chambers 459 or 461. Except for the openings mentioned, the inbushing 413, into the second chamber 461 and out 7 through opening 451 into the outlet tube 433, preferably the same size as tube 435, which delivers the gas to the large chamber 427 which acts as a compliance. The gas leaves the unit 401 through the outlet bushing 416 which is preferably the same diameter as the inlet bushing 413. The inlet and outlet tubes 435 and 443 act as minor inertances but are primarily connecting and spacing tubes (corresponding to tubes 321 and 325 of FIG. 8) for the compliances 423 and 427 with respect to the inertance 441. The actual length of the path followed by a particle of gas through the unit 401 is a part of the length of the exhaust system and the location of the compliance and inertance sections in the system preferably follows the disclosure set forth above. This determines the lengths of tubes 435 and 443 with the sizes of the various chambers being selected to give the desired degree of attenuation. 7

FIGS. 10 11 show another modifed and more compact form of in line pulse converter 501. In this device, there is a three part outer housing comprising a rectangular inlet end section 503, a rectangular outlet end section 505, and an intermediate or central section ofv circular cross section 507. The center section 507 has portions extending inwardly into each of the end sections 503 and 505 and is in contact with and spotwelded to top and bottom portions of the end sections 503 and 505 along lines of tangency as indicated at 509. The inner portions of the end sections 503 and 505 are either bent inwardly or provided with partitions as seen at 51 1 and 513 in contact with the shell 507 and suitably secured as by welding to the shell to provide a gas tight transition section joint between the end shell sections and the center shell section 507.

The outer end of the inlet shell section 503 has a transverse header 515 and a transverse-reinforcement partition 517 and, similarly, the outer end of the outlet section 505 has a transverse header 519 and a transverse reinforcement partition 521, the partitions and headers being spotwelded to the shells 503 and 505. The headers and partitions have aligned necks 523 which receive and support the inner ends of an inlet bushing 525 and an outlet bushing 527, the bushings being longitudinally aligned along the center line of the unit 501.

The center section shell 507 has an outer portion 529 on the upstream side which projects along a substantial part of the length of the inlet shell 503 and defines with the shell 503 a passage 531 that is located between the outer periphery of the-shell portion 529 and the inner periphery of the shell section 503. A transverse imperforate partition 533 is spotwelded in place within the portion 529 of the shell 507 and blocks direct flow from the inlet bushing 525 through the shell section 507 and forces flow through the peripheral passage 531. Located on the downstream side of the partition 533 and adjacent to it are a pair of diametrically opposite round holes 535 which connect the passage 531 with the interior of the shell 507.

In a similar fashion, the shell section 507 has a portion 537 that projects into and along most of the length of the outlet shell 505 so that its outer periphery defines with the inner periphery of the section 505 a peripheral passage 539. The shell 507 has an imperforate spot-welded transverse partition 541 located in it and within the section 505. Immediately upstream and adjacent to the partition 541 there are a pair of preferably round holes 543 (similar to holes 535) which connect the peripheral passage 539 with the inside of the shell 507.

At approximately a midpoint between the partitions 511 and 513, the center shell 507 has a transverse partition 545 which is spotwelded to it and which has a center neck 547, preferably aligned with bushings 525 and 527. Neck 547 supports an imperforate openended tube 549, preferably at the midpoint of the tube, which is preferably of the same internal diameter as the inlet and outlet bushings.

The space 551a inside the section 503 upstream of the section 507, together with the space 551k inside of the section 507 upstream of the partition 533 comprise a volume or compliance chamber 551 and, similarly, the space 553a inside of the section 505 and downstream of the section 507, together with the space 553b inside of the section 507 and downstream of the partition 541 comprise a volume or a compliance chamber 553, the volumes 551 and 553 preferably being the same. The peripheral passages 531 and 539 act as inertances and because of the particular area location the flow and pressure distribution is improved as compared with an ordinary tube, such as 23, to give higher and better quality inertance and improved performance, especially at higher flows. The small chamber 555 between the partitions 533 and 545 and the small chamber 557 between the partitions 545 and 541 form small or minor compliances that act with the inertance tube 549 to form an overall major inertance that connects the volumes 551 and 553, being similar to the double can arrangement 51 and 305 shown in previous figures.

The cross sectional area of the passages 531 and 539 and the combined cross sectional areas of the two openings 535 and the two openings 543 are preferably mentary converter 9 of FIG. 1, the more efficient double can inertance converters 51, 123, 301, and 401 of FIGS. 4, 6, 8, and 9, and the still more efficient double can-peripheral inertance converter of FIGS. 10-11. It is noted that the various volumes .or compliances are not utilized as expansion chambers in this system so that instead of the common ten or so to one expansion ratio'for the expansion mode, ratios of less than five to one (cross sectional area of chamber to cross sectional area of conduit) are used. The double can inertance section is not only enhanced as an inertance but contains three reactive elements (as compared with one in FIG. 1) and 'this'substantially increases the number of antiresonant frequencies available from various multiples and combinations of individual elements so that they can be cascaded across a wide band of consistently deep attenuation. The converter 501 adds two more efficient inertances for substantial further enhancement of this effect.

The converters are designed to remove all the high energy high frequencies at an upstream point of the exhaust system as close as practicable to the engine. The cutoff frequency is preferably about the third or fourth harmonic so that the fundamental and three or four lower frequencies pass through the converter with some but not complete attenuation and with a pulse shape that can be readily handled by acoustic control devices located downstream of the converter, such as such as rotary engine exhaust systems. In relatively low pressure applications, such as four cycle engine exhaust systems, self-plugging of the holes is less likely to occur and holes or louvers in the conduits may be used to increase inertance and attenuation. For minimum backpressure, it is preferred that the cross sectional area of the conduits or flow passages through the converter be no less than that of the inlet bushing, and this feature appears inconverters 123, 301, 401, and 501 The relative compliance diagram of FIG. 3 is very useful to the silencing system designer. In a sense it is like a standing wave diagram but where the standing wave diagram shows the critical points fora single frequency, the compliance diagram shows them for entire pulse or all the notes and noise together. It therefore reveals the optimum locations to attenuate all the notes and noise. It is believed that at the-low compliance points (e.g., 60 inches, inches, and inchesin FIG. 3) the pipe appears as necked down to the pulse and by placing compliances at these points, the backpressure may be lowered even below what it would be for a straight pipe. This is, obviously, a valuable feature in small engine applications where minimum backpressure is needed for adequate performance.

The compliance diagram not only shows where to center the compliances (at the lows) and the inertances (at the highs), but gives useful information on the size of the components since the spacing between'peaks gives an indication of what the lengths of the elements should be. For example, assuming that a midportion of a compliance should be at 60 inches the opposite ends are spaced on either side so that they are vertically aligned with points part way up the curves. With this approach, a 10 inch length with midpoint at 60 inches 100 inches or I45 inches appears feasible and proved to be very effective as indicated above. The

length of the double caninertance (e.g., 305 in FIG. 8).

can be estimated in the same way. Having determined the lengths of the inertance and compliances, the lengths of the connecting tubes (and minor inertances) is simply that required to interconnect them at the proper locations. The preferred size of the exhaust conduit is a known (e.g., 2 inches in the system discussed above) and the optimum diameter of the compliance can be estimated by assuming that it has a cross sectional area approximately in the range of three to five times that of the conduit. Generally, starting upstream,

compliances and inertances are put in at, the low and 1 1 high peaks until the pulses are converted to shapes and characteristics that can be silenced by normal silencing techniques used in conventional automotive exhaust systems, i.e., the standing wave approach.

While the invention is of particular value in connection with exhaust systems for rotary engines (e.g., the Wankel engine), two cycle engines, and low noise level four cycle engines, it may be used in other gas or fluid flow systems. For example, the present pulse converters provide beneficial pulse conversion when used in a like manner with 4 cycle reciprocating piston engines, or other sources of pulsating gas flow such as compressors, etc., producing pulses containing as few as eight to individual high pressure noise producing frequencies. Such engines generally have large area, rapid opening exhaust ports and may produce pulses aslow as 160 db (about 0.20 psi). However, pulse converters of this invention became of most significant practical effectiveness and are most aptto be economically feasible when used in conjunction with sound sources producing gas pulses containing at least high energy frequencies and having sound pressure at or in excess of 0.25 psi. At or above these limits dependence upon conventional silencing components is generally prohibited by space limitations and cost penalties due to the unrealistic numbers of such components required. Modifications in the particular structures shown may be made without departing from the spirit and scope of the invention.

We claim:

1. In an internal combustion engine or the like, an exhaust system connected to said engine to receive exhaust gas discharged by the engine, said system having a series of predetermined maximum and minimum acoustical compliance points spaced along the length of the system, a pulse converter located in said system and having an inlet receiving exhaust gas from the engine and an outlet discharging gas into the remainder of the exhaust system, said converter comprising in series between said inlet and outlet a first compliance section, an inertance section, and a second compliance section, said compliance sections being spaced longitudinally apart in the system and located substantially at minimum compliance points in said system and said inertance section being located substantially at a maximum compliance point in the system.

2. A system according to claim 1 including silencing means in the system downstream of the converter for attenuating the converted pulse discharged by the converter.

3. In an internal combustion engine or the like discharging exhaust gas pulses containing at least about 20 high energy frequencies to be attenuated and having sound pressure in the order ofO.25 psi or higher, an exhaust system connected to said engine to receive gas discharged by the engine, a pulse converter located in said system and having an inlet receiving exhaust gas from the engine and an outlet, said converter serving to remove a band of high frequencies from said pulses and convert them to converted pulses containing substantially only the fundamental frequency and no more than about three low harmonics of said fundamental, and silencing means in said system connected to said converter outlet for attenuating said fundamentaland harmonics.

4. A system as set forth in claim 3 wherein said converter inlet and outlet have substantially the same cross sectional area and said converter comprises a first volume means attached to said inlet and a second volume means attached to said outlet to provide first and second volumes through which gas flows in series, said volumes having cross sectional areas no more than about five times that of said inlet and outlet, and an acoustical inertance means serving to extend between and attach the first volume means to the second volume means.

5. A system as set forth in claim 4 wherein said first and second volume means are spaced apart a distance such that they are substantially centered in a lengthwise dimension on adjacent minimum relative compliance points for said system.

6. A system as set forth in claim 5 wherein said inertance means is substantially centered on a maximum acoustical compliance point for said system.

7. In a pulsating gas fiow system having an inlet and an outlet and a predetermined length between said inlet and outlet and having predetermined high and low points of acoustical compliance located along said length, a pulse converter in'said system, said converter comprising a first volume forming a first compliance section, a second volume forming a second compliance section, an inertance member, means for gas flow connecting the inertance member to the first volume, means for gas flow connecting the inertance member to the second volume, said volumes said means and said inertance member all being arranged so that gas flows directly through the volumes and means and inertance member in series flow, said first volume and said second volume being spaced apart by a distance corre sponding to the distance between selected points of low compliance in said system and being substantially centered on said points, said inertance member being spaced between said compliance sections and located so that it is substantially centered on a high point of compliance.

8. A system as set forth in claim 7 wherein said converter attenuates frequencies above a substantially predetermined cutoff frequency, said system including a muffler downstream of said converter and tuned to attenuate frequencies below said cutoff frequency.

9. In a gas flow system containing acoustical pulsations such as the exhaust system of an internal combustion engine, said system having a predetermined point of maximum acoustical compliance located within its length, said system containing means for attenuating audible sound in acoustic pulses entering the system, said means including an acoustical inertance device located in said system substantially at said point of maximum compliance, said inertance device comprising an elongated tubular housing having a transverse inner partition subdividing the housing into two volumes, a longitudinally extending gas flow tube supported in said partition and extending into and opening into both of said volumes for conducting gas from one volume to the other, said housing having an inlet opening in one volume and an outlet opening into the other volume each located so that gas flows through a substantial part of its respective volume between the opening and an end of said gas flow tube.

10. An inertance device as set forth in claim 9 wherein said housing has an inner cross sectional area substantially in the range of three to five times the cross sectional area of the gas flow tube.

11. An inertance device as set forth in claim wherein said inlet and outlet have substantially the same cross sectional areas as said gas flow tube.

12. An inertance device as set forth in claim 9 wherein said gas flow tube is imperforate and substantially centered longitudinally on said partition and said partition is substantially centered longitudinally in said housing and said housing is substantially centered longitudinally on said point of maximum compliance.

13. An inertance device as set forth in claim 9 wherein said inlet and outlet comprises tubular passages substantially coaxial with said gas flow tube.

14. An inertance device as set forth in claim 9 wherein said tubular housing has transverse walls at opposite ends and bushings mounted in said transverse walls in longitudinal alignment with said gas flow tube and providing said inlet and outlet.

15. An inertance device as set forth in claim 9 wherein said tubular housing has transverse walls on opposite sides of said partition and acting with the partition to define said volumes and said inlet and outlet are located in the sidewall of the housing adjacent the respective transverse walls.

16. An inertance device as set forth in claim 15 including a substantially L-shaped inlet tube attached at one end to said housing around said inlet and extending substantially parallel to and coextensive with said housing and a substantially L-shaped outlet-tube attached at one end to said housing around said outlet and extending substantially parallel to and coextensive with said housing.

17. An inertance device as set forth in claim 9 wherein said housing comprises a tubular shell, said shell having transverse walls on opposite sides of said partition and acting with the partition .to define said volumes, said inlet and outlet comprising holes located in said shell adjacent the respective transverse walls, and at least one elongated gas flow passage of substantially annular cross sectional shape opening into at least one of said inlet and outlet holes and forming an acoustical inertance in addition to that provided by the housing and gas flow tube.

18. An inertance device as set forth in claim 17 wherein said gas flow tube, said inlet and outlet, and said gas flow passage have substantially equal cross sectional areas.

19. An inertance device as. set forth in claim 9 wherein said housing comprises a tubular shell, said shell having-transverse walls an opposite sides of said partition spaced inwardly substantial distances from the outer ends of the shell and acting with the partition to define said volumes, said inlet and outlet comprising holes located in said shell adjacent the respective transverse walls, a tubular inlet end shell telescoped over the inlet end of said tubular shell and defining therewith a substantially annular inlet gas passage communicating with said inlet, a tubular outlet end shell telescoped' over the outlet end of said tubular shell and defining therewith a substantially annular outlet gas passage communicating with said outlet, the spaces inside outer ends of said tubular shell opening into the spaces inside said respective end shells and forming compliance volumes opening into said respective inlet and outlet gas passages.

20. In a system as set forth in claim 9, means forming compliance volumes in series gas flow relationship with said inertance device, said compliance volumes and inlencing means in the system located upstream of said compliance volumes and inertance device and comprising a length of perforated straight through gas flow tube and a sound absorbing chamber containing sound absorbing material surrounding the perforated length of said straight through tube, said chamber and material acting in conjunction with said perforated length of tube to attenuate the highest sound frequencies in the gas pulses entering the system.

22. In a fluid flow system having a predetermined length and having an inlet end connected to an engine or the like for producing an acoustically pulsating flow of fluid and having an outlet end opening into atmosphere, said system having predetermined high points of acoustic compliance and predetermined low points of acoustic compliance spaced from each other along the length of the system, said system having a conduit connected to said engine and of a predetermined diameter, an elongated tubular housing located in said system and having an inlet and an outlet located at opposite longitudinal ends of .the housing, said inlet being connected to said conduit, said tubular housing being substantially centered longitudinally on one of said points and having a cross sectional area which is in the range of three to five times the cross sectional area of said conduit, and conduit means connected to the outlet of said housing for conducting gas to the outlet of said system.

23..ln a fluid flow system having a predetermined length and having an outlet opening to atmosphere and an inlet connected to a source of acoustically pulsating fluid such as an engine, said system having predetermined points located along its length of minimum acoustic compliance, a pair of housings in said system of tubular elongated shape having an inlet and an outlet spaced from each other at opposite ends of the housings so that gas flows along the length of the housing, said housings being located at and longitudinally centered upon said points of minimum compliance in the system, and conduit means interconnecting the outlet of the first housing and the inlet of the second housing and serving to space them apart in accordance with the spacing between said points of minimum compliance.

24. A system as set forth in claim 23 including an inlet conduit in the system connected to said engine and of a predetermined diameter, said housings having substantially uniform cross sectional areas substantially in the range of three to five times the cross sectional area of said' conduit.

25'. In an exhaust system for a Wankel type rotary engine producing shock wave type exhaust acoustic pulses containing more than one hundred audible high pressure notes to be attenuated in each pulse, the combination of a first conduit connected to receive the exhaust gas discharged by the engine, a straight through flow silencing device connected to said first conduit and having a perforated straight through flow passage, a housing surrounding and forming with said perforated conduit a high frequency sound attenuating chamber,

sound absorbing means in said chamber for attenuating the highest frequencies in said pulses, a pulse converter device connected to receive gas passed through said silencing device, said pulse converter device comprising compliance chambers and an inertance section located between the compliance chambers, said pulse converter serving to remove substantially all frequencies in the gas pulses above about the third harmonic, and a sound attenuating muffler connected to receive gas that has passed through the pulse converter, said muffler containing silencing means for attenuating the audible frequencies in the pulses discharged by the pulse converter, and conduit means connecting the outlet side of said muffler to atmosphere.

26. A pulse converter for attenuating a broad range of high decibel notes in acoustic pulses of flowing gas comprising first chamber means forming a first volume, second chamber means forming a second volume, each chamber means having a gas inlet to the volume and a gas outlet to the volume and said inlet and outlet being separated so that gas flows through the volume in going from inlet to outlet, third chamber means forming a third volume and containing a partition subdividing said third volume into adjacent but separated first and second sub-volumes, said third chamber meanshaving a gas inlet to the first sub-volume and a gas outlet to the second sub-volume, an open ended gas flow tube sup ported in and extending through said partition and projecting into each of said sub-volumes and providing a path for gas to flow from the first sub-volume to the second sub-volume, the gas inlet to the first sub-volume and the gas outlet to the second sub-volume being separated from the ends of the gas flow tube so that gas flows through the sub-volumes, a first gas flow passage means connecting the outlet of the first chamber means to the inlet of the third chamber means, and a second gas flow passage means connecting the outlet of the third chamber means to the inlet of the second cham-' ber means, said first and second gas flow passage means and said gas flow tube being of cross sectional areas throughout their entire lengths which are several times less than the cross sectional areas of any of said volumes and sub-volumes.

27. A pulse converter as set forth in claim 26 wherein said first and second gas passage means and said gas flow tube are impcrforate.

28. A pulse converter as set forth in claim 26 wherein said chamber means comprise tubular metal shells and metal walls at opposite ends of the shells, and metal transverse pressure resisting reinforcement partitions secured inside the shells adjacent the end walls to provide double wall constructions at the ends of the shells.

29. A pulse converter as set forth in claim 26 wherein said first. second, and third chamber means comprise separated axially aligned tubular shells and said first and second gas passage means comprise pipes axially aligned with said shells.

30. A pulse converter as set forth in claim 26 wherein said first and second chamber means comprise a common tubular shell and transverse partitions in said shell to form the first volume adjacent one end of the shell and the second volume adjacent the other end of the shell.

31. A pulse converter as set forth in claim 30 wherein said third chamber means comprises an inner tubular shell within said common shell and supported on said transverse partitions, and said first and second gas flow passage means comprise pipes within said common shell and supported on said transverse partitions.

32. A pulse converter as set forth in claim 31 wherein said pipes are substantially L-shaped and connect to the side of said inner tubular shell.

33. A pulse converter as set forth in claim 26 wherein said first, second, and third chamber means are substantially coaxial, said third chamber means comprising a central elongated tubular shell and said first and second chamber means comprising end tubular shells telescoped over the respective ends of the central shell and defining with the outer periphery of said central shell said first and second gas passage means.

34. A pulse converter as set forth in'claim 33 wherein said first and second volumes-are formed in part by inner end portions of said central shell in combination with inner portions of said end shells.

35. A pulse converter as set forth in claim 34 wherein said central shell is substantially circular in cross section and said end shells are substantially rectangular in cross section and tangent to opposite sides of said central shell.

36. A pulse converter set forth in claim 35 wherein the inlet and outlet to the third chamber means comprise holes in said central shell opening into ends of said first and second gas passage means.

37. A pulse converter as set forth in claim 36 wherein said inlet and outlets, said gas flow tube, and said first and second gas passage means all have substantially uniform and identical cross sectional areas throughout their lengths.

38. A pulse converter as set forth in claim 37 wherein said gas flow tube is impcrforate.

39. A pulse converter as set forth in claim 26 wherein said first, second, and third chamber means are axially aligned and comprise a commontubular shell containing a plurality of transverse partitions dividing the interior of said shell into said volumes and sub-volumes.

40. A pulse converter as set forth in claim 39 wherein said first and second gas passage means comprise pipes supported on said partitions.

41. A method of testing for the points of high and low compliance in the acoustic pulsations of a gas flow system having a predetermined length which comprises attaching a uniform diameter conduit of said predetermined length to the source of said acoustically pulsating gas flow to provide a uniform diameter passage for gas to flow the length of the system, measuring the peak pulse pressure and pulse width at numerous points along the length of said conduit while gas is flowing through the conduit at a desired rate of flow to provide comparative values for the ratio of change. in pulse width to change in pulse pressure between adjacent points along the length of said conduit, said comparative values indicating the relative acoustic compliance for said points.

42. A method as set forth in claim 41 including the step of causing gas to flow through said conduit at a rate that causes substantially the highest sound pressure in normal operation of the source and making said measurements when the gas flows at said rate.

43. In a method of attenuating audible notes in acoustic pulsations of a gas flow system of predetermined length, the steps of measuring the peak pulse pressures and pulse widths at numerous points along the length of the system and utilizing such measurements to locate high and low compliance peaks in the system, placing inertance means in the system at one or more of the high peaks, and placing compliance means in the system at two or more low peaks.

44. in a method of attenuating sound associated with the acoustic pulsations in a gas flow system by means of compliance volumes and inertances, the steps of attaching a predetermined length of uniform diameter conduit to the source of said pulsating gas flow to provide a uniform diameter passage for gas to flow the length of the system utilizing a microphone and oscilloscope to obtain peak voltage and pulse width information at each of numerous points along the length of the conduit while gas is flowing through the conduit at a desired rate of flow, plotting said information on a graph wherein the ratio of the change in pulse width between adjacent points to the change in pulse voltage between said adjacent points is plotted as one coordinate and the point location is the other coordinate, placing an inertance device in said system at the location of one of the high points on said graph, and placing compliance volumes in said system at the location of two or more of the low points on said graph.

45. in a method of attenuating sound associated with the acoustic pulsations in a gas flow system by means of acoustic compliance volumes and acoustic inertances, the steps of attaching a predetermined length of uniform diameter longitudinally extending conduit to the source of said acoustically pulsating gas flow to provide a uniform diameter passage for gas to flow the length of the system, utilizing a microphone and oscilloscope to obtain peak voltage and pulse width information at each of numerous points along the length of the conduit while gas is flowing through the conduit at a desired rate of flow, said information revealing the maximum and minimum ratios of the change in pulse width between adjacent points to the change in pulse voltage between said adjacent points and the longitudinal points at which said ratios are located, placing an inertance device in said system at the location of one of the maximum ratio points, and placing compliance volumes in said system at the locationof two or more of the minimum ratio points.

46. In an internal combustion engine or the like discharging shock wave exhaust gas pulses containing at least' about high energy frequencies to be acoustically attenuated and having sound pressure in the order of 10 psi, an exhaust system connected to said engine to receive gas discharged by the engine, a pulse converter located in said system and having an inlet receiving exhaust gas from the engine and an outlet, said converter serving to remove a band of high frequencies from said pulses and convert them to converted pulses containing substantially only the fundamental frequency and no more than about three low harmonics of said fundamental, and silencing means in said system connected to said converter outlet for attenuating said fundamental and harmonics. 

1. In an internal combustion engine or the like, an exhaust system connected to said engine to receive exhaust gas discharged by the engine, said system having a series of predetermined maximum and minimum acoustical compliance points spaced along the length of the system, a pulse converter located in said system and having an inlet receiving exhaust gas from the engine and an outlet discharging gas into the remainder of the exhaust system, said converter comprising in series between said inlet and outlet a first compliance section, an inertance section, and a second compliance section, said compliance sections being spaced longitudinally apart in the system and located substantially at minimum compliance points in said system and said inertance section being located substantially at a maximum compliance point in the system.
 2. A system according to claim 1 including silencing means in the system downstream of the converter for attenuating the converted pulse discharged by the converter.
 3. In an internal combustion engine or the like discharging exhaust gas pulses containing at least about 20 high energy frequencies to be attenuated and having sound pressure in the order of 0.25 psi or higher, an exhaust system connected to said engine to receive gas discharged by the engine, a pulse converter located in said system and having an inlet receiving exhaust gas from the engine and an outlet, said converter serving to remove a band of high frequencies from said pulses and convert them to converted pulses containing substantially only the fundamental frequency and no more than about three low harmonics of said fundamental, and silencing means in said system connected to said converter outlet for attenuating said fundamental and harmonics.
 4. A system as set forth in claim 3 wherein said converter inlet and outlet have substantially the same cross sectional area and said converter comprises a first volume means attached to said inlet and a second volume means attached to said outlet to provide first and second volumes through which gas flows in series, said volumes having cross sectional areas no more than about five times that of said inlet and outlet, and an acoustical inertance means serving to extend between and attach the first volume means to the second volume means.
 5. A system as set forth in claim 4 wherein said first and second volume means are spaced apart a distance such that they are substantially centered in a lengthwise dimension on adjacent minimum relAtive compliance points for said system.
 6. A system as set forth in claim 5 wherein said inertance means is substantially centered on a maximum acoustical compliance point for said system.
 7. In a pulsating gas flow system having an inlet and an outlet and a predetermined length between said inlet and outlet and having predetermined high and low points of acoustical compliance located along said length, a pulse converter in said system, said converter comprising a first volume forming a first compliance section, a second volume forming a second compliance section, an inertance member, means for gas flow connecting the inertance member to the first volume, means for gas flow connecting the inertance member to the second volume, said volumes said means and said inertance member all being arranged so that gas flows directly through the volumes and means and inertance member in series flow, said first volume and said second volume being spaced apart by a distance corresponding to the distance between selected points of low compliance in said system and being substantially centered on said points, said inertance member being spaced between said compliance sections and located so that it is substantially centered on a high point of compliance.
 8. A system as set forth in claim 7 wherein said converter attenuates frequencies above a substantially predetermined cutoff frequency, said system including a muffler downstream of said converter and tuned to attenuate frequencies below said cutoff frequency.
 9. In a gas flow system containing acoustical pulsations such as the exhaust system of an internal combustion engine, said system having a predetermined point of maximum acoustical compliance located within its length, said system containing means for attenuating audible sound in acoustic pulses entering the system, said means including an acoustical inertance device located in said system substantially at said point of maximum compliance, said inertance device comprising an elongated tubular housing having a transverse inner partition subdividing the housing into two volumes, a longitudinally extending gas flow tube supported in said partition and extending into and opening into both of said volumes for conducting gas from one volume to the other, said housing having an inlet opening in one volume and an outlet opening into the other volume each located so that gas flows through a substantial part of its respective volume between the opening and an end of said gas flow tube.
 10. An inertance device as set forth in claim 9 wherein said housing has an inner cross sectional area substantially in the range of three to five times the cross sectional area of the gas flow tube.
 11. An inertance device as set forth in claim 10 wherein said inlet and outlet have substantially the same cross sectional areas as said gas flow tube.
 12. An inertance device as set forth in claim 9 wherein said gas flow tube is imperforate and substantially centered longitudinally on said partition and said partition is substantially centered longitudinally in said housing and said housing is substantially centered longitudinally on said point of maximum compliance.
 13. An inertance device as set forth in claim 9 wherein said inlet and outlet comprises tubular passages substantially coaxial with said gas flow tube.
 14. An inertance device as set forth in claim 9 wherein said tubular housing has transverse walls at opposite ends and bushings mounted in said transverse walls in longitudinal alignment with said gas flow tube and providing said inlet and outlet.
 15. An inertance device as set forth in claim 9 wherein said tubular housing has transverse walls on opposite sides of said partition and acting with the partition to define said volumes and said inlet and outlet are located in the sidewall of the housing adjacent the respective transverse walls.
 16. An inertance device as set forth in claim 15 including a substantially L-shaped inlet tube attached at one end to said housing aRound said inlet and extending substantially parallel to and coextensive with said housing and a substantially L-shaped outlet-tube attached at one end to said housing around said outlet and extending substantially parallel to and coextensive with said housing.
 17. An inertance device as set forth in claim 9 wherein said housing comprises a tubular shell, said shell having transverse walls on opposite sides of said partition and acting with the partition to define said volumes, said inlet and outlet comprising holes located in said shell adjacent the respective transverse walls, and at least one elongated gas flow passage of substantially annular cross sectional shape opening into at least one of said inlet and outlet holes and forming an acoustical inertance in addition to that provided by the housing and gas flow tube.
 18. An inertance device as set forth in claim 17 wherein said gas flow tube, said inlet and outlet, and said gas flow passage have substantially equal cross sectional areas.
 19. An inertance device as set forth in claim 9 wherein said housing comprises a tubular shell, said shell having transverse walls an opposite sides of said partition spaced inwardly substantial distances from the outer ends of the shell and acting with the partition to define said volumes, said inlet and outlet comprising holes located in said shell adjacent the respective transverse walls, a tubular inlet end shell telescoped over the inlet end of said tubular shell and defining therewith a substantially annular inlet gas passage communicating with said inlet, a tubular outlet end shell telescoped over the outlet end of said tubular shell and defining therewith a substantially annular outlet gas passage communicating with said outlet, the spaces inside outer ends of said tubular shell opening into the spaces inside said respective end shells and forming compliance volumes opening into said respective inlet and outlet gas passages.
 20. In a system as set forth in claim 9, means forming compliance volumes in series gas flow relationship with said inertance device, said compliance volumes and inertance having a cutoff frequency at about the fourth harmonic above the fundamental frequency of the system, and silencing means in the system downstream of said compliance volumes and inertance substantially attenuating audible sound in the pulses having frequencies below about said fourth harmonic.
 21. In a system as set forth in claim 20 including silencing means in the system located upstream of said compliance volumes and inertance device and comprising a length of perforated straight through gas flow tube and a sound absorbing chamber containing sound absorbing material surrounding the perforated length of said straight through tube, said chamber and material acting in conjunction with said perforated length of tube to attenuate the highest sound frequencies in the gas pulses entering the system.
 22. In a fluid flow system having a predetermined length and having an inlet end connected to an engine or the like for producing an acoustically pulsating flow of fluid and having an outlet end opening into atmosphere, said system having predetermined high points of acoustic compliance and predetermined low points of acoustic compliance spaced from each other along the length of the system, said system having a conduit connected to said engine and of a predetermined diameter, an elongated tubular housing located in said system and having an inlet and an outlet located at opposite longitudinal ends of the housing, said inlet being connected to said conduit, said tubular housing being substantially centered longitudinally on one of said points and having a cross sectional area which is in the range of three to five times the cross sectional area of said conduit, and conduit means connected to the outlet of said housing for conducting gas to the outlet of said system.
 23. In a fluid flow system having a predetermined length and having an outlet opening to atmosphere and an inlet connected to a source of acoustically pulsating fluid such as an engine, said system having predetermined points located along its length of minimum acoustic compliance, a pair of housings in said system of tubular elongated shape having an inlet and an outlet spaced from each other at opposite ends of the housings so that gas flows along the length of the housing, said housings being located at and longitudinally centered upon said points of minimum compliance in the system, and conduit means interconnecting the outlet of the first housing and the inlet of the second housing and serving to space them apart in accordance with the spacing between said points of minimum compliance.
 24. A system as set forth in claim 23 including an inlet conduit in the system connected to said engine and of a predetermined diameter, said housings having substantially uniform cross sectional areas substantially in the range of three to five times the cross sectional area of said conduit.
 25. In an exhaust system for a Wankel type rotary engine producing shock wave type exhaust acoustic pulses containing more than one hundred audible high pressure notes to be attenuated in each pulse, the combination of a first conduit connected to receive the exhaust gas discharged by the engine, a straight through flow silencing device connected to said first conduit and having a perforated straight through flow passage, a housing surrounding and forming with said perforated conduit a high frequency sound attenuating chamber, sound absorbing means in said chamber for attenuating the highest frequencies in said pulses, a pulse converter device connected to receive gas passed through said silencing device, said pulse converter device comprising compliance chambers and an inertance section located between the compliance chambers, said pulse converter serving to remove substantially all frequencies in the gas pulses above about the third harmonic, and a sound attenuating muffler connected to receive gas that has passed through the pulse converter, said muffler containing silencing means for attenuating the audible frequencies in the pulses discharged by the pulse converter, and conduit means connecting the outlet side of said muffler to atmosphere.
 26. A pulse converter for attenuating a broad range of high decibel notes in acoustic pulses of flowing gas comprising first chamber means forming a first volume, second chamber means forming a second volume, each chamber means having a gas inlet to the volume and a gas outlet to the volume and said inlet and outlet being separated so that gas flows through the volume in going from inlet to outlet, third chamber means forming a third volume and containing a partition subdividing said third volume into adjacent but separated first and second sub-volumes, said third chamber means having a gas inlet to the first sub-volume and a gas outlet to the second sub-volume, an open ended gas flow tube supported in and extending through said partition and projecting into each of said sub-volumes and providing a path for gas to flow from the first sub-volume to the second sub-volume, the gas inlet to the first sub-volume and the gas outlet to the second sub-volume being separated from the ends of the gas flow tube so that gas flows through the sub-volumes, a first gas flow passage means connecting the outlet of the first chamber means to the inlet of the third chamber means, and a second gas flow passage means connecting the outlet of the third chamber means to the inlet of the second chamber means, said first and second gas flow passage means and said gas flow tube being of cross sectional areas throughout their entire lengths which are several times less than the cross sectional areas of any of said volumes and sub-volumes.
 27. A pulse converter as set forth in claim 26 wherein said first and second gas passage means and said gas flow tube are imperforate.
 28. A pulse converter as set forth in claim 26 wherein said chamber means comprise tubular metal shells and mEtal walls at opposite ends of the shells, and metal transverse pressure resisting reinforcement partitions secured inside the shells adjacent the end walls to provide double wall constructions at the ends of the shells.
 29. A pulse converter as set forth in claim 26 wherein said first, second, and third chamber means comprise separated axially aligned tubular shells and said first and second gas passage means comprise pipes axially aligned with said shells.
 30. A pulse converter as set forth in claim 26 wherein said first and second chamber means comprise a common tubular shell and transverse partitions in said shell to form the first volume adjacent one end of the shell and the second volume adjacent the other end of the shell.
 31. A pulse converter as set forth in claim 30 wherein said third chamber means comprises an inner tubular shell within said common shell and supported on said transverse partitions, and said first and second gas flow passage means comprise pipes within said common shell and supported on said transverse partitions.
 32. A pulse converter as set forth in claim 31 wherein said pipes are substantially L-shaped and connect to the side of said inner tubular shell.
 33. A pulse converter as set forth in claim 26 wherein said first, second, and third chamber means are substantially coaxial, said third chamber means comprising a central elongated tubular shell and said first and second chamber means comprising end tubular shells telescoped over the respective ends of the central shell and defining with the outer periphery of said central shell said first and second gas passage means.
 34. A pulse converter as set forth in claim 33 wherein said first and second volumes are formed in part by inner end portions of said central shell in combination with inner portions of said end shells.
 35. A pulse converter as set forth in claim 34 wherein said central shell is substantially circular in cross section and said end shells are substantially rectangular in cross section and tangent to opposite sides of said central shell.
 36. A pulse converter as set forth in claim 35 wherein the inlet and outlet to the third chamber means comprise holes in said central shell opening into ends of said first and second gas passage means.
 37. A pulse converter as set forth in claim 36 wherein said inlet and outlets, said gas flow tube, and said first and second gas passage means all have substantially uniform and identical cross sectional areas throughout their lengths.
 38. A pulse converter as set forth in claim 37 wherein said gas flow tube is imperforate.
 39. A pulse converter as set forth in claim 26 wherein said first, second, and third chamber means are axially aligned and comprise a common tubular shell containing a plurality of transverse partitions dividing the interior of said shell into said volumes and sub-volumes.
 40. A pulse converter as set forth in claim 39 wherein said first and second gas passage means comprise pipes supported on said partitions.
 41. A method of testing for the points of high and low compliance in the acoustic pulsations of a gas flow system having a predetermined length which comprises attaching a uniform diameter conduit of said predetermined length to the source of said acoustically pulsating gas flow to provide a uniform diameter passage for gas to flow the length of the system, measuring the peak pulse pressure and pulse width at numerous points along the length of said conduit while gas is flowing through the conduit at a desired rate of flow to provide comparative values for the ratio of change in pulse width to change in pulse pressure between adjacent points along the length of said conduit, said comparative values indicating the relative acoustic compliance for said points.
 42. A method as set forth in claim 41 including the step of causing gas to flow through said conduit at a rate that causes substantially the highest sound pressure in normal operation of the source and making said measurements when thE gas flows at said rate.
 43. In a method of attenuating audible notes in acoustic pulsations of a gas flow system of predetermined length, the steps of measuring the peak pulse pressures and pulse widths at numerous points along the length of the system and utilizing such measurements to locate high and low compliance peaks in the system, placing inertance means in the system at one or more of the high peaks, and placing compliance means in the system at two or more low peaks.
 44. In a method of attenuating sound associated with the acoustic pulsations in a gas flow system by means of compliance volumes and inertances, the steps of attaching a predetermined length of uniform diameter conduit to the source of said pulsating gas flow to provide a uniform diameter passage for gas to flow the length of the system utilizing a microphone and oscilloscope to obtain peak voltage and pulse width information at each of numerous points along the length of the conduit while gas is flowing through the conduit at a desired rate of flow, plotting said information on a graph wherein the ratio of the change in pulse width between adjacent points to the change in pulse voltage between said adjacent points is plotted as one coordinate and the point location is the other coordinate, placing an inertance device in said system at the location of one of the high points on said graph, and placing compliance volumes in said system at the location of two or more of the low points on said graph.
 45. In a method of attenuating sound associated with the acoustic pulsations in a gas flow system by means of acoustic compliance volumes and acoustic inertances, the steps of attaching a predetermined length of uniform diameter longitudinally extending conduit to the source of said acoustically pulsating gas flow to provide a uniform diameter passage for gas to flow the length of the system, utilizing a microphone and oscilloscope to obtain peak voltage and pulse width information at each of numerous points along the length of the conduit while gas is flowing through the conduit at a desired rate of flow, said information revealing the maximum and minimum ratios of the change in pulse width between adjacent points to the change in pulse voltage between said adjacent points and the longitudinal points at which said ratios are located, placing an inertance device in said system at the location of one of the maximum ratio points, and placing compliance volumes in said system at the location of two or more of the minimum ratio points.
 46. In an internal combustion engine or the like discharging shock wave exhaust gas pulses containing at least about 100 high energy frequencies to be acoustically attenuated and having sound pressure in the order of 10 psi, an exhaust system connected to said engine to receive gas discharged by the engine, a pulse converter located in said system and having an inlet receiving exhaust gas from the engine and an outlet, said converter serving to remove a band of high frequencies from said pulses and convert them to converted pulses containing substantially only the fundamental frequency and no more than about three low harmonics of said fundamental, and silencing means in said system connected to said converter outlet for attenuating said fundamental and harmonics. 